The present invention relates to a friction material, in particular, a friction material that is used for industrial machinery, railway vehicles, luggage vehicles and passenger vehicles, more specifically brake pads, brake linings and clutch facings that are used in the above applications.
The friction material that is mainly used in brakes and so on is reinforced with a fiber to improve the strength. As the reinforcement fiber, as asbestos-substituting fiber, glass fiber, steel fiber, aramid fiber and potassium titanate fiber are used. Since these have respective characteristics, these are used in combinations of several kinds thereof.
Among these fibers, potassium titanate fiber is highly evaluated as abrasive inorganic fiber that can not only improve the strength of the friction material as a whole, the heat resistance and the wear resistance, but also improve the coefficient of friction of the friction material owing to the abrasiveness thereof, in particular, can secure high coefficient of friction during high load such as high-speed braking.
With an intention to provide a friction material excellent in the fading resistance and the resistance to squeaking, in patent literature 1 (:JP-A-2000-272978), a brake friction material in which potassium titanate whisker having high heat resistance and appropriate hardness is compounded is disclosed.
However, since the whisker is needle-like single crystals having a diameter in the range of 0.5 to 1 μm, a length in the range of 10 to 20 μm, that is, a high aspect ratio, from a viewpoint of work environment health, it is not desirable to use.
As a countermeasure to this, with an object of obtaining a friction material that, without using whisker-like potassium titanate that is unfavorable from a viewpoint of work environment health, can impart the same friction action and strength and can suppress a rise of manufacturing cost, patent literature 2 (:JP-A-2001-172612) discloses a friction material in which powdery or scaly potassium titanate having an aspect ratio of 3 or less is compounded, patent literature 3 (:JP-A-2001-020986) discloses a friction material where potassium titanate powder is compounded, and patent literature 4 (JP-A-2003-082331) discloses a friction material in which scaly or planar potassium titanate having an aspect ratio not exceeding 3 is compounded.
However, when, as in the above patent literatures 2 through 4, in place of potassium titanate whisker, non-whisker-like potassium titanate such as columnar, planar or scaly potassium titanate is compounded in a friction material, there is a problem in that since the porosity is lowered, resultantly, the fading characteristics are deteriorated and the high-speed effectiveness is lowered. For instance, test results of the fading characteristics of examples 1, 3, 6 and 10 in Table 1 of the patent literature 4 are shown with mark Δ that shows a unsatisfying result of min μ>0.20.
Furthermore, in addition to the above object, with another object of obtaining a friction material in which in dust generated owing to friction with mating member during use fine fibers are not mingled, patent literature 5 (JP-B-08-032601) discloses a friction material in which flaky polycrystalline fiber of a titanium compound is compounded.
However, the potassium titanate whisker disclosed in the patent literature 1 and the non-whisker-like potassium titanate such as columnar, planar, scaly and flaky ones disclosed in the patent literatures 2, 3, 4 and 5 all are higher in the melting point than plain cast iron that is the material of the mating member. Accordingly, in particular, in the case of a temperature of a friction surface rising as during high-load braking, there are problems in that the above materials transfer to the mating member, and the squeaking sound and wear of the friction material result.